The present invention relates to an electric double layer capacitor of high electrostatic capacitance which utilizes the principles of an electric double layer that is composed of an interface between activated carbon and electrolyte.
Recently, high-capacitance capacitors based on the electric double layer principles have been developed as backup power supplies for memories in electronic systems, and are widely used with microcomputers and IC memories.
One type of electric double layer capacitor is disclosed in U.S. Pat. No. 3,536,936, for example. The electric double layer capacitor disclosed therein comprises a frame-shaped gasket, a pair of polarized electrodes made of activated carbon particles and disposed in the gasket, a separator disposed between the polarized electrodes for preventing electrons from moving between the electrodes, and a pair of current collectors mounted respectively on the opposite surfaces of the polarized electrodes and having peripheral edges bonded to the gasket. The polarized electrodes are made as paste electrodes from a concentrated slurry which is a mixture of powdery or particulate activated carbon and an electrolyte.
It is important that the internal resistance of such an electric double layer capacitor with paste electrodes be low. The internal resistance of an electric double layer capacitor is greatly affected by the contact resistance of active carbon of the polarized electrodes and the contact resistance between the collector electrodes and the polarized electrodes.
Therefore, in order to reduce the internal resistance of the polarized electrodes and the contact resistance between the collector and polarized electrodes, each basic cell of the electric double layer capacitor should be kept under vertical pressure to bring the particles of the paste activated carbon into good electric contact with each other. Conventional electric double layer capacitors require each cell to be kept under a pressure of about 100 kg/cm.sup.2 though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used. In prior electric double layer capacitors, the cells are kept under pressure by deforming the outer cases of the capacitors or bonding the current collectors strongly to gaskets. If an electric double layer capacitor is to be used as a large-capacitance capacitor, e.g., a power supply for energizing a motor, then it is necessary to increase the cross-sectional areas of the electrodes of the basic cell. Therefore, the pressure to be applied to the basic cell has to be increased. Increasing the pressure, however, causes some practical problems such as the selection of means for applying the pressure and the need for high rigidity for the outer cell which houses the basic cell.
The contact resistance between the polarized electrodes and the current collectors of conventional electric double layer capacitors is so large that it is responsible for presenting a high internal resistance. One proposed electric double layer capacitor which has a reduced contact resistance between the polarized electrodes and the current collectors and hence a reduced internal resistance is disclosed in Japanese Laid-Open Patent Publication No. 63(1988)-232309.
According to the disclosure of the above publication, a thermoplastic resin is heated and dissolved in an electrolytic solution, and the solution is cooled into a gel which does not flow or is not deformed and remains in a solid phase unless an external force is applied thereto. Then, powder of activated carbon is added to the gel, and the mixture is kneaded by a roll mill. The kneaded mixture is shaped into a sheet which is cut off into polarized electrodes. Metallic powder of stainless steel is sprinkled over one surface of each of the polarized electrodes, and the sprinkled powder particles are embedded in the surface of the polarized electrode by being pressed by a roll, thus providing an electrically conductive layer which will reduce the contact resistance between the polarized electrode and a current collector. Since the electrically conductive layer is formed after the polarized electrode is fabricated, however, an increased number of processing steps are required to manufacture the electric double layer capacitor. In addition, the contact resistance between the polarized electrode and the electrically conductive layer is not satisfactory enough since they are electrically coupled to each other through contact between the particles.